Space Explorations- The Black Hole Chapter
One of the most fascinating and mysterious members of the universe is the black hole. It’s a mystery no one has been able to comprehend completely so far. Black holes can be visualized as bottom-less holes in the fabric of space-time (“Into a Black Hole”-S.W.Hawking,2008). Black holes were predicted by Einstein’s Theory of Relativity, a mass inside its own Schwarzschild radius. When a star dies, it leaves behind a massive core which, if is more than three times as massive as the sun, gets converted into a black hole.
Many great scientists have worked on this topic. It was concluded that any black hole process moves in the direction of increasing area, when two black holes merge, their combined area can never be smaller than the sum of their individual areas before combination. This is analogous with the second law of thermodynamics which states that all processes are accompanied by an increase in entropy. Thus, area can be visualized as the entropy equivalent for black holes (Bekenstein, 1973). Jacob Bekenstein’s paper ( Physical Review D volume 7 issue 8) provides a beautiful insight on studying black hole physics through the lens of thermodynamics. Black hole thermodynamics…a magnificent topic.
The holographic principle (‘t Hooft; Susskind; Bousso, 2002), the information paradox (Hawking, Perry, Strominger, 2016) …there are a lot of hidden secrets yet to be uncovered about black holes. Things would be so much easier if we could just see them.
Nothing can escape a black hole, not even light, and hence it’s impossible to observe it. However, its effect on surrounding matter can be observed. Being a highly massive substance in a small volume, its gravitational field is immense. So, when it passes through a cluster of interstellar matter, the matter will be sucked into it via a process known as accretion. This will emit X-rays which can be captured by devices. Attempts to try and decode a black hole are being made in this manner.
In typical telescopes, light reflects from a metal mirror and refracts from a lens. X-rays however pass straight through them without getting focused. Thus, the resolution is poor. The NuSTAR (Nuclear Spectroscopic Telescope Array) mission launched the first orbiting telescope to focus light in the high energy X-ray region and resolve this issue.
NuSTAR has two ‘eyes’ at one end of the telescope, each of which contains 135 concentric shells called glancing incidence mirrors which ensure that the X-rays get concentrated into the other end of the telescope where X-ray detectors are present. These two structures are separated by a long beam (Space.com).
This telescope is of immense help in studying black holes in other galaxies, the black hole in the center of the Milky way, supernovae and other interesting astrophysical phenomena and structures. Many exciting facts and explanations have been discovered already.
Supernovae are formed when a star is out of fuel to burn, collapses onto itself and bounces back. No concrete model has been in place to fully explain this, even computer models stall after the bounce back. There are two possible theories- Either an extra source of energy is breaking the stall or the star’s spin creates a jet that will break the stall.
NuSTAR is equipped to detect Titanium which is emitted while supernovae are formed. If the first theory is right, then on observing a supernova, a ring of titanium must be observed, else a line of titanium along the must be observed. Observations of Cassiopeia A, a supernova in the Milky Way which exploded 500 years ago revealed blobs of titanium. From this evidence, along with new theoretical models developed by other science teams, NuSTAR scientists were able to conclude that the material inside of a star “sloshes” around inside, creating enough energy to break the stall and explode (Daniel Stern, interview, inverse.com).
The NuSTAR also focused on studying the supernova 2014J in the M82 galaxy where a brightly emitting X-ray source was found in the galactic disk. The data collected revealed that it was pulsating. The act requires immense magnetic field; hence it couldn’t be a black hole. However, it could be a neutron star. It was also found to be accreting at hundreds of times the Eddington Limit. The Eddington Limit is the maximum possible luminosity of a star so that its electrons don’t get thrown out of the sphere. The researchers had just discovered the first ultra-luminous pulsar (Daniel Stern, interview, inverse.com).
A major area which NuSTAR is trying to understand is regarding the shape of the black hole’s corona and why it flares up. Astronomers believe that there is another source of emission near a black hole disk called the corona. Coronas are made of highly energetic particles; they generate X-ray light. But details about coronas — how they form, what they look like, even where they are exactly it the region surrounding the hole — are unclear.
There are two models proposed for the structure of the corona. The “lamppost” model says they are compact light sources, that sit above and below the black hole, along its rotation axis. The other model proposes that the coronas are spread out more diffusely, either as a larger cloud around the black hole, or as a “sandwich” that envelops the surrounding disk of material (cited in nasa.gov).
Observations of the active black hole at the center of the galaxy Markarian 335, which lies roughly 350 million light-years away in the constellation Pegasus by Dan Wilkins (Saint Mary’s University, Canada) and colleagues made them think that the corona spreads over the accretion disk first, then gathers itself together, contracts into a vertical, jet-like structure and launches off the disk at about 20% of the speed of light. Though the corona was clearly giving away X-rays, they were hardly reflected from the disk. This can be explained by the principle of relativistic beaming (Ubachukwu & Chukwude). If the source of radio emission is moving near the speed of light along a direction which lies close to the line of sight, then the source nearly catches up with its own radiation. As the velocity of the corona increases, more of its radiation is relativistically beamed away and lesser radiation is reflected by the accretion disk. The flare ends when the corona collapses back into the disk. Thus, a clear idea of how flares happen has been provided.
The black hole is no more as mysterious as it used to be. We are starting to get clues as to how it behaves. There’s still a lot more to be explained and we have a good head start. Further observations in the future will provide more revelations regarding the various aspects of the black hole, more advanced technologies and missions will contribute to this. Maybe we may get a chance to journey into one too, who knows? Space explorations contribute highly to various fields- physics, communication, military services and a lot more. Space exploration is one of the biggest achievements of humans as a whole, it has paved the way to greater scientific understanding and many technological advancements over the years and will do so too in the future.
Bibliography
· http://www.skyandtelescope.com/astronomy-news/how-black-holes-flare-0511201523/
· http://www.mathpages.com/home/kmath339/kmath339.htm
· http://www.hawking.org.uk/into-a-black-hole.html
· https://www.space.com/15109-nasa-nustar-mission-black-holes.html
· https://www.inverse.com/article/31948-nustar-changed-know-about-universe
· https://www.nasa.gov/feature/jpl/nustar/black-hole-has-major-flare
· Black holes and entropy, Jacob Bekenstein, Apr 15th 1973 Physical Review D volume 7 issue 8 pp 2333–2346 DOI: 10.1103/PhysRevD.7.2333
· The holographic principle, Raphael Bousso Institute for Theoretical Physics, University of California, Santa Barbara, California 93106, U.S.A.∗
· Soft Hair on Black Holes- Stephen W. Hawking†, Malcolm J. Perry† and Andrew Strominger∗ ; †DAMTP, Centre for Mathematical Sciences, University of Cambridge, Cambridge, CB3 0WA UK ∗ Center for the Fundamental Laws of Nature, Harvard University, Cambridge, MA 02138, USA
· On the Relativistic Beaming and Orientation Effects in Core-Dominated Quasars, A. A. Ubachukwu∗ & A. E. Chukwude Department of Physics and Astronomy, University of Nigeria Nsukka, Nigeria ∗Regular Associate of the Abdus Salam ICTP, Trieste Italy.
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About:
I am currently in my first year at IIT Madras, pursuing the B.Tech Engineering Physics programme. A quirky, funny, physics and music enthusiast who’s a passionate debater and orator, musician, and a lover of sports and writing, I love trying out many things. Perseverance, honesty and dedication are some of the qualities I value the most and try my best to be so.
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This article was written while interning under Dr.Tafheem Masudi and Dr.Sukant Khurana
Originally published at medium.com on April 25, 2018.
